error handling in cast functions for user defined types

Hi,

I'm writing a set of casts to/from various user defined
types.  As is unexpected, there are cases where one
data type doesn't neatly map to another (for certain
values).  In these cases I emit an INVALID_PARAMETER_VALUE
or OUT_OF_RANGE error -- depending on the situation.

But, should I also PG_RETURN_NULL()?  I note the
cast of int4's to int2's signal similar errors -- *but*
returns "(int2) value".

Is the return value just *ignored* when the error is
signaled?

Thanks!
--don